In ion mobility spectrometry devices, separation of gas-phase ions is accomplished by exploiting variations in ion drift velocities under an applied electric field arising from differences in ion mobility. One well-known type of ion mobility spectrometry device is the High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) cell, also known by the term Differential Ion Mobility Spectrometry (DMS) cell, which separates ions on the basis of a difference in the mobility of an ion at high field strength (commonly denoted as Kh) relative to the mobility of the ion at low field strength (commonly denoted as K). Briefly described, a FAIMS cell comprises a pair of spaced apart electrodes that define therebetween a separation region through which a stream of ions is directed. An asymmetric waveform comprising a high voltage component and a lower voltage component of opposite polarity, together with a DC voltage (referred to as the compensation voltage, or CV) is applied to one of the electrodes. When the ion stream contains several species of ions, only one ion species is selectively transmitted through the FAIMS cell for a given combination of asymmetric waveform peak voltage (referred to as the dispersion voltage, or DV) and CV. The remaining species of ions drift toward one of the electrode surfaces and are neutralized. The FAIMS cell may be operated in single ion detection mode, wherein the DV and CV are maintained at constant values, or alternatively the applied CV may be scanned with time to sequentially transmit ion species having different mobilities. FAIMS cells may be used for a variety of purposes, including providing separation or filtering of an ion stream prior to entry into a mass analyzer.
FIG. 1 schematically depicts a first known system 100 for analyzing ions that includes a FAIMS device 155. A solution of sample to be analyzed is introduced as a spray of liquid droplets into an ionization chamber 105 via atmospheric pressure ion source 110. Ionization chamber 105 is maintained at a high pressure relative to the regions downstream in the ion path, typically at or near atmospheric pressure. Atmospheric pressure ion source 110 may be configured as an electrospray ionization (ESI) probe, wherein a high DC voltage (either positive or negative) is applied to the capillary or “needle” through which the sample solution flows. Other suitable ionization techniques may be utilized in place of ESI, including without limitation such well-known techniques as atmospheric pressure chemical ionization (APCI), heated electrospray ionization (HESI), and thermospray ionization.
Ions produced by the ion source enter the FAIMS cell 155 through an aperture 117 in an entrance plate 120 and then through an inlet orifice 150 after passing through an expansion chamber 111. The expansion chamber is provided with a gas, typically helium or other inert gas, which is introduced into the expansion chamber 111 via a gas conduit 113. A portion of the gas flows back into the ionization chamber 105 through entrance plate aperture 117 in counter-flow to the ions and droplets and serves to desolvate charged droplets. Another portion of the gas combines with the analyte ions in chamber 111 and serves as a carrier gas through the FAIMS cell 155. The combined ion/carrier gas flow then enters FAIMS cell 155 through inlet orifice 150. The carrier gas flow may be carefully metered to maintain flow rates within predetermined limits which will depend on the FAIMS cell size, electrode geometry, and operational considerations. An electrical potential difference is maintained between the entrance plate 120 and the FAIMS cell 155 and, thus, physical separation is maintained between these components. Accordingly, a non-conducting sealing element 173, such as a gasket or O-ring maintains the FAIMS gas within the apparatus and prevents contamination of this gas from outside air. Because of drawing-space limitations, this sealing element is not explicitly shown in some of the accompanying drawings.
Generally speaking, the FAIMS cell 155 includes inner and outer electrodes 165 and 170 having radially opposed surfaces, which define therebetween an annular separation region 175 (an “analytical gap”) through which the ions are transported. The FAIMS cell geometry depicted in FIG. 1, as well as in other figures herein may be generally referred to as a “side-to-side FAIMS cell”, in which the longitudinal axes (axes of cylindrical surfaces, directed out of the page) of inner electrode 165 and outer electrode 170 are oriented transversely with respect to the overall direction of ion flow. The principles of the design and operation of FAIMS cells and other ion mobility spectrometry devices have been extensively described elsewhere in the art (see, for example, U.S. Pat. No. 6,639,212 to Guevremont et al., incorporated by reference herein in its entirety), and hence will not be described in detail herein. In brief, the carrier gas and ions flow through the separation region 175 from inlet orifice 150 to exit orifice 185. Ion separation is effected within the separation region (analytical gap) 175 of the FAIMS cell 155 by applying an asymmetric waveform having a peak voltage (DV) and a compensation voltage (CV) to one of the inner or outer electrodes, 165, 170. The values of CV and DV are set to allow transmission of a selected ion species through separation region 175. Other ion species having different relative values of high field and low field mobilities will migrate to the surface of one of the electrodes and be neutralized.
Still referring to FIG. 1, the selected ions emerge from the FAIMS cell 155 through exit orifice 185 and pass through a small gap 183 separating the FAIMS cell 155 from a mass spectrometer 157. Whereas most of the carrier gas exhausts through the gap 183 at atmospheric pressure, ions are electrostatically guided into at least one reduced pressure chamber 188 of the mass spectrometer 157 through an orifice in the mass spectrometer or through an ion transfer tube 163. The at least one reduced pressure chamber may be evacuated by a vacuum port 191. At least a portion of ion transfer tube 163 may be surrounded by and in good thermal contact with a heat source, such as heater jacket 167. The heater jacket 167, which may take the form of a conventional resistance heater, is operable to raise the temperature of ion transfer tube 163 to promote further desolvation of droplets entering the ion transfer tube 163.
From the at least one reduced pressure chamber 188, ions are transferred through an orifice 193 of a skimmer 194 into a high vacuum chamber 195 maintained at a low pressure (typically around 100 millitorr) relative to the reduced pressure chamber 188. The high vacuum chamber 195 is typically evacuated by turbo or similar high-vacuum pumps via a vacuum port 197. The skimmer 194 may be fabricated from an electrically conductive material, and an offset voltage may be applied to skimmer 194 to assist in the transport of ions through interface region and into skimmer orifice 193. Ions passing through skimmer orifice 193 may be focused or guided through ion optical assembly 198, which may include various electrodes forming ion lenses, ion guides, ion gates, quadrupole or octopole rod sets, etc. The ion optical assembly 198 may serve to transportions to an analyzer 199 for mass analysis. Analyzer 199 may be implemented as any one or a combination of conventional mass analyzers, including (without limitation) a quadrupole mass analyzer, ion trap, or time-of-flight analyzer.
Computational and experimental studies have shown that the gas stream carrying ions from the ion source enters the FAIMS separation region 175 with high velocity. This high velocity gas flow causes the gas stream (including a significant portion of the ions) to impinge onto the portion of the inner electrode 165 that directly faces the inlet orifice 150 prior to turning into the analyzer gap, thus discharging a significant percentage of the ion population of interest on the inner electrode. An angular gas stream flowing out of the entrance plate and into the source region may also skew and misalign the ion beam with the FAIMS entrance, thereby partially steering the ion beam onto the entrance plate causing further ion loss.
The computational and experimental studies reveal poor ion transmission from the ion source to the exit of FAIMS. e.g. a transmission of approximately 10% for bromochloroacetate anion [BCA, having a mass-to-charge ratio, m/z, of 173]. For example, FIG. 2 shows the results of calculated simulations of ion trajectories of the BCA anion within the FAIMS 155, with the ion-cloud region 127 indicating the region within which most of the ions flow. The simulations shown in FIG. 2, which include simulations of ion flow within a flowing gas, indicate that significant ion losses occur at the entrance plate 120 and on a portion of the inner electrode 165 that is exposed to the FAIMS inlet 150. Curve 204 of FIG. 3 illustrates the results of a simulated CV scan of the BCA anion through the known FAIMS apparatus of FIG. 2. This may be compared with curve 202, which presents the results of a second simulation in which ions are hypothetically introduced between the FAIMS electrodes, without encountering the entrance plate. The overall transmission of ions from the ion source to the exit orifice is only approximately 10%. Although ions are lost near the FAIMS entrance, few ions are lost inside the analytical gap 175 because, owing to the cylindrical shape of the FAIMS electrode (FIG. 1), ions within the separation region 175 that is away from the inlet orifice 150 experience a non-uniform electric field which causes spatial focusing of ions that are travelling in the gap. This ion focusing results in only minimal ion loss of the selected ion species during transport as a result of diffusion or the separation field. In accordance with the above considerations, there is a need in the art of ion transport and analysis for improved means for controlling the effects of gas flow on ion trajectories.